Apparatus for extremely rapid determination of ionization and appearance potentials in a mass spectrometer



P. MARTIGNom ETAL 3,493,742 FOR EXTR Feb. 3, 1970 APPARATUSy EMELY RAPIDDETERMINATION 0F IONIZATION AND APPEARANCE POTENTIALS IN A MASSSPECTROMETER 4 Sheets-Sheet l Fi-led Sept. 16, 1965 Feb- 3, 1970 P.MARTIGNONI ETAL 3,493,742

APPARATUS FOR EXTREMELY RAPID DETERMINATION I OF IONIZATION ANDAPPEARANCE POTENTIALS IN A MASS SPECTROMETER Filed Sept. 16. 196i 4Sheets-Sheet 2 TRAP l SH EL I 29 I l.I: I-I |I5,=L]/ 3 l0N 25 ANoDE I uI7 2| 24 CATHODS mum I BACKING II V`:\7 I \3| PLATE #nl I5 I9 27 5/ =I|IauNcI-II-:o IoNs `23 'Re v :F' WI 4 PUMP CONTROL i 9 30/ UNIT`\vACUuIIII l: FIG' 4 SIDE 5 CYCLES /sEc. I 7 I I souARING AMPLIFIERIoKcl FLIP FLOPS I 4 4 I L J 5 CYELE`S/gEC -1 42 I l.:

I I 4I I I T 2 5 e Ie 3 2 s4 I 45 I |"\37 l EMITTER FoLLowERs^ v V Y lTO ANALOG POTENTIOMETERS I L I Pasquale MartIgnonI Robert L. Morgan LeeF. McClung FIG. 2 Henryi A. NappIer Char es M. Cason III INVENTOII 4Sheets-Sheet 5 Pasquale Martignon Robert L. Morgan Lee F. McClun'eHenrr'ANa pler Char es M. CpasonJlI 1NVENTORS. MAN;- 777.

)m @ML P. MART-IGNONI ETAL APPARATUS FOR EXTREMELY RAPID DETERMINATION0F IONIZATION AND APPEARANCE POTENTIALS IN A MASS SPECTROMETER Feb.3.1970

Filed sept. le, 1955 ION CURRENT INPUT FIG. v3

Feb- 3, 1970 P. MARTIGNONI ETAI.

'APPARATUS FOR EXTREMELY RAPID DETERMINATION OF IONIZATION ANDAPPEARANCE POTENTILS `IN A MASS SPECTROMETER Filed Sept. 16, 196,5

NUMBER OF ELECTRONS 4 Sheets-Sheet 4 FIG. 5A

FIG. 5B

I AE

\\ Aie ENERGY I/gcglofcgI/IPONENT ALONG DIRECTION oF P (1s-qualeMangnoni oberr L. Morgan FIG. 5C I ,ee F. McCIun e Henr' A. NcIppIerChor es M. Cason .III

INVENTO t. )wwf m 1W BY Lfm J. W )W BML WCM United Smtes Patent Office3,493,742 Patented Feb. 3, 1970 3,493,742 APPARATUS FOR EXTREMELY RAPIDDETERMI- NATION OF IONIZATION AND APPEARANCE POTENTIALS IN A MASSSPECTROMETER Pasquale Martignoni, Huntsville, Ala., Robert L. Morgan,Fayetteville, Tenn., Lee F. McClune, What Cheer, Iowa, and Henry A.Nappier, Lacey Springs and Charles M. Cason III, Huntsville, Ala.,assignors to the United States of America as represented by theSecretary of the Army Filed Sept. 16, 1965, Ser. No. 487,937

Int. Cl. H01j 39/34 U.S. Cl. Z50-41.9 11 Claims ABSTRACT OF THEDISCLOSURE A time of flight mass spectrometer is automaticallycontrolled by voltage drivers which are in turn controlled by astaircase voltage from a frequency divider in accordance with desiredconditions.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

The invention relates generally to the automation of a technique fordetermining the ionization or appearance potentials of a compound byusing a mass spectrometer. Specifically the present invention relates tothe replacement of all manual controls of Foxs Retarding PotentialDifference technique by automatic electronic controls. Also thisinvention provides for the plotting directly of the ionizationetliciency curve.

Foxs Retarding Potential Difference (R.P.D.) technique is generallyregarded as one of the most accurate and sensitive techniques used toobtain ionization (IP) and appearance potentials (AP). It is, however,also one of the most frustrating, time consuming and laborioustechniques. Many variables have to be controlled and set up by hand tocarry out the R.P.D. technique. The trap current must be maintainedconstant while varying the electron energy control grid and repellergrid voltages in a linear pre-selected manner. The plots must becompared and their difference plotted to get the desired (IF curves).Doing all this manually requires a considerable amount of time. Further,the element of human error is ever present-especially after many hoursof doing the same task.

It is therefore an object of this invention to provide a system forautomatically determining the ionization and appearance potentials of acompound.

A further object of the present invention is to replace the manualcontrols of Foxs R.P.D. technique with automatic electronic controls.

A still further object of the invention is to provide for the plottingdirectly of a final ionization efiiciency curve in which AI is plottedversus E.

The present invention relates to a modification of Foxs R.P.D.technique, in which all the variables are electronically controlled. Thedata is automatically analyzed and the results are plotted on an x-yrecorder. The total time for a complete determination is 3.8 sec. Thecurves are accurate, reproducible, and show fine details. Thesensitivity of this system is such that IPs may be obtained from waterand oxygen background peaks when the total pressure in the massspectrometer is between 6x10*8 and 1x10-9 torr. The electronic apparatusis divided into two major divisions: the control loop and the processingloop. The control loop functions to maintain the trap current constantwhile varying the electron energy control grid and repeller gridvoltages in a linear pre-selected manner. The processing loop takes avoltage proportional to an ion current and compares this voltage withthe one obtained from the previous increment of electron energy andrepeller grid voltage. This ion current difference signal is thenplotted as a function of the electron energy; giving one an ionizationefficiency curve.

The invention further resides in and is characterized by various novelfeatures of construction, combinations, and arrangements of parts whichare pointed out with particularity in the claims annexed to and forminga part of this specification. Complete understanding of the inventionand an introduction to other objects and features not specificallymentioned will be apparent to those skilled in the art to which itpertains when reference is made to the following detailed description ofa specific embodiment thereof and read in conjunction with the appendeddrawing. The drawing, which forms a part of the specification, presentsthe same reference characters to represent corresponding and like partsthroughout the drawing, and wherein:

FIGURE l shows a functional block diagram of the invention;

FIGURE 2 is a schematic diagram illustrating the binary electronics ofthe present invention;

FIGURE 3 shows a schematic illustration of the spectrometer controlcircuit and control data processing circuit of this invention;

FIGURE 4 is a schematic showing of a time of flight mass spectrometerand its connections to the system; and

FIGURES 5A, 5B and 5C are waveforms illustrating the electron energypresent in the ionization chamber of the spectrometer.

The study of the ionization (IP) and appearance (AP) potentialsresulting from electron impact on various gaseous molecules has been ofconsiderable scientific interest. The IP is an important property ofmolecules and has been used in the interpretation of electronic andmolecular structures. The AP has, also, been extremely useful in thedetermination of bond energies and electronegativities and as a measureof reactivity in organic reactions. The AP can be treated as an ordinarychemical reaction so that for the reaction,

where Hf is the thermodynamic heat of formation and E is excess energyformed in the process. An equivalent statement is where D is the bondstrength of A-B.

A considerable number of techniques are currently in use to determinethe IP and AP by electron impact with a mass spectrometer. The best one,and the one used in our system, is the Retarding Potential Differencemethod (R.P.D.). The R.P.D. technique consists of introducing an extragrid (the repeller grid 1 of FIGURE 4) between the filament 3 and theionization chamber 5. The potential of repeller grid 1 is made slightlynegative with respect to filament 3 to provide a sharp low energycut-off for the electron beam 7. This can be better understood byreferring to FIGURES 5A, 5B and 5C. FIGURE 5A shows the energydistribution of electrons emitted from the filament. It may be notedthat the energy distribuution is exponential on the repeller grid. Asharp energy cut-off in the low range is achieved as shown in FIGURE 5B.When the retarding potential of the repeller grid is changed by a smallamount a slice of electrons having only a small change in electronenergy AE is repelled. See FIGURE 5C. If the change in the voltage onthe repeller grid is made very small, then the energy band AE of theelectrons can be considered to be monoenergetic beam of electrons(within the limits of the given apparatus). The difference in the ioncurrent caused by the two conditions of the repeller grid is measured.It can readily be seen from FIGURES 5B and 5C that any change in the ioncurrent can only be caused by the beam of electrons having the energyband AE. Whenthe repeller grid is changed to give one, FIGURE 5C, thereare two possibilities for the behavior of the ion current: First, nochange; second, a small change. If the potential between the controlgrid 9 and the ionization region 5 (FIGURE 4) is less than thatnecessary for ionization of the molecules under investigation, no changein the intensity of the ion current occurs. If the potential differenceis greater than the ionization potential, a change in ion current equalto the ions formed by the slice (AE) of electrons is noted. Byincreasing the potential between the ionization region and grid 9 bysmall increments (also the initial potential of the repeller grid 1 isincreased by the same amount) and noting the difference in the ioncurrent caused by changing grid 1 by the small amount, a measure of theion current as a function of energy AE is obtained.

In order to better understand the operation of the system set forth inFIGURE 1, a description of its specific components and their functionsis first presented.

In the mass spectrometer 10` (shown schematically in FIGURE 4) a sampleof the gas from the compound to be analyzed is placed in the ionizationchamber and kept at a particular pressure by apparatus not shown. Theions are produced by bombardment of the sample with pulsed electron beam7. The beam is pulsed at the rate of 10,000 times per second. Electronsare made available by continuous emission from a hot wire filament 3.Control grid 9 is biased negatively with respect to the lament, thuspreventing electrons from passing continuously into the ionizing region.When control grid 9 is pulsed positively, a burst of electrons leavesthe filament area and passes through the narrow slits in the grids andforms a beam in the chamber 5 which is directed towards the electrontrap. Grids 4 and 6 are focusing grids. Molecules of the sample are nowbombarded by the electrons and break down into positive and negativeions and neutral radicals. These ions will collect in the ionizingregion 5.

Immediately after the electron beam is shut off, by making control grid9 negative again, the ion focus grid 15 is pulsed negatively to withdrawpositive ions. The ions are attracted toward this grid and into theaccelerating region 17. Ions entering this region are very stronglyattracted to the highly negative ion energy grid 19 giving them animpulse of kinetic energy such that they are di rected down a field-freedrift tube 21 towards a collector 23. Since all ions receive an equalenergy impulse, their respective velocities vary according to theirmass-tocharge ratio. Since all ions leave the accelerating regionpractically simultaneously and are allowed to drift some distance priorto striking the collector, those of equal mass will tend to bunch andcollectively separate from non-similar mass bunches. The lighter masses22 have higher velocities than the heavier masses 24. As each bunchstrikes the ion cathode 23, electrons are knocked from its surface andattracted to the slot between the field strip 25 and the dynode strip 27of multiplier 29. Proper voltage levels are provided by control unit 30.Electrons cycloid down the multiplier and are collected on the anode 31producing a negative voltage pulse which is the output of the massspectrometer and represents the ion current flow.

FREQUENCY DIVIDER Frequency divider 35 is shown in detail in FIGURE 2.The frequency divider continuously divides the operational frequency ofthe spectrometer (10 kc.) by 2X103. This action generates an output of 5cycles/second. The frequency divider consists of an eleven binarycounter which adds Set inputs to the 8 and 16 counters when the 512counter is in the true state. This addition of 24 counts to the chaineffectively subtracts 24 counts during the second cycle of the counterthereby causing it to emit an output pulse for every thousand countsinstead of every 1024 counts. These counters need not be reset sincethey continually give division by 2X103. The division by 2X1()3determines the switching time of the repeller grid 1. This time isautomatically divided by 2 by the 1 flip-flop of the digital to analogconverter 37; therefore the energy step time increment 11-0 is adivision of the 10l kc. by 4X103. With this arrangement, each electronenergy step will be long enough for four thousand ion trains to driftdown the flight tube and be analyzed. Two thousand ion trains areproduced with the repeller grid at some voltage, approximately 0.5 v.with respect to the electron energy. The next two thousand ion trainswill be produced with the repeller grid switched to 0.6 v. Thedifference between the number of ions in these bunches is equivalent tothe number of ions which would be produced by all the electrons of AEfrom 0.5 to 0.6 in electron energy distribution function. The presentsystem has one hundred steps of increasing electron energy. The voltageon the repeller grid is repeatedly switched from negative to morenegative as described with each of these steps.

DIGITAL TO ANALOG CONVERTER The digital to analog converter 37 is shownin detail in FIGURES 2 and 3. It contains AND gate 39, flip-flop 41, andswitch 42 to connect and disconnect counter 44. The converters sevenstage binary counter 41, whose input is taken from the output of the ANDgate 39 automatically further divides the frequency by 2. The outputs ofeach binary stage are fed by an amplifier to its correspondingpotentiometer shown in FIGURE 3. The potentiometers are set in the sameratio of impedance as the value of the stage with which it isassociated.

The output from the frequency divider is applied to one input of an ANDgate 39, while its other input is supplied by the output of the singlestage bistable multivibrator or ip-op 41. To start, single stage flip-op41 is set true by momentarily closing the switch 42 between it and thesquaring amplifier 4.3 contained in the frequency divider. When tiip-op41 is set, its output signal to the input of the AND gate is true andfive cps. output from the frequency divider is counted on the sevenstage register. The 1, 4, 32, and 64 flip-flop have their not trueoutputs connected to 0R gates 45 and 47 while the other flip-flops ofthe counter 41 have their true outputs connected to OR gate 45. Withthese connections, there will be no change in the output from OR gate 47until only flip-fiops 1, 4, 32, and 64 are set and flip-flops 28" and 16are reset. Therefore, OR gate 47 only resets ip-fiop 41 after a count of100. When flip-flop 41 is reset by OR gate 47, AND gate 39 loses one ofits inputs and removes the frequency divider signal from the seven stageregister 44.

ELECTRON ENERGY DRIVER The electron energy driver 51 is shown in FIGURE3 as having an operational amplifier 53 which accepts three inputs: thevoltage staircase 113 generated by the digital to analog converter 37;the initial electron energy setting circuit 55; and the output of thetrap current regulator 57.

CONTROL GRID DRIVER The control grid driver 59 (FIGURE 3) containsamplifier 61 which accepts two inputs: a fixed input, grid bias 63,which is used to adjust the initial bias; and the output from thedigital to analog converter. The control grid and electron energydrivers work together in keeping a constant bias voltage on the controland electron energy electrodes. The voltage staircase generated by thedigital to analog converter raises the energy of the ionizing elec- 5trons by discrete steps which have been determined by the settings ofthe analog potentiometers.

REPELLER GRID DRIVER In FIGURE 3 the repeller grid driver 65 is shown ashaving an operational amplifier 67 which accepts three inputs: one inputfrom potentiometer 69 is used to clamp the signal 110 from the frequencydivider; a second input is the 5 cps. output of the frequency divider;and the third input is taken from the digital to analog converter and isused to step the repeller grid voltage simultaneously with the electronenergy and grid bias voltages. The potentiometer 69 determining thesecond input is adjusted so the frequency divider signal is to -0.1volt. The fixed voltage potentiometer 69 is adjusted to give +0.05 volt.This combination generates a i005 volt output from the amplifier 67which drives the repeller grid.

TRAP CURRENT REGULATOR The trap current regulator 57 takes its voltagesignal directly from a cathode follower meter `driver 73. This voltageis divided by two at amplifier 75 (shown in FIG- URE 3). The phase ofone of the inputs is reversed to eliminate a 150 v. potential from themass spectrometer. The output of amplifier 77 represents the cathodedifference voltage of the meter driver. The output of amplifier 77 isapplied to the input of amplifier 53 of electron energy driver 51 andthus regulates the electron energy to a preset value as the trap currentregulator automatically forces the difference output of the meter driverto zero.

DATA PROCESSOR In FIGURE 3 the data processor 79 is shown in detail. The5 cycle output from frequency divider 35 is fed to amplifier 80.Amplifier 80 and amplifier 82, the inverter, drive diode switches 84 and85 of the switch amplifiers 86 and 88 as well as the sample and holdamplifiers 91 and 93. The -phase of the 5 cycle signal is so adjustedthat the switch amplifier 86 and the sample and hold amplifier 91 are onwhen switch amplifier 88 and the sample and hold amplifier 93 are off.This switch voltage must be larger than the largest signal fed to allthese amplifiers; otherwise, it will not determine the switching ofthese amplifiers. It will be noted that the switches are driven insynchronism with the repeller grid and also that the switches willoccupy both of their two possible states during each step of theelectron energy.

The operation of the data processor begins at the onset of the ionizingvoltage at the initial ionization potential, the repeller grid voltageis high by the AE where the AE is determined by the potentiometersetting. The ion current signal output 95 of spectrometer 10 (seeFIGURE 1) at this electron energy is applied to amplifier 97 and switchamplifier 86 simultaneously. The output of amplifier 97 tries to driveswitch amplifier 88, but the diode switch 84 is reset by the 5 cps.source. Switch amplifier 86 being Set has an inverter output equal tothe magnitude of the input voltage. The output of amplifier 86 drivesamplifier 100 which serves as an inverter and as a zero adjustment forthe output of amplifier 86. The output of amplifier 100 drives thesample and hol amplifier 91 which is in the sample state and acts as again of 1 inverter. The output of amplifier 91 drives amplifier 97 whichalso has the same ion current signal thereon. The output voltage fromamplifier 91 is inverted and equal in magnitude to the ion current inputsignal. The output of amplifier 97 is zero when the experiment firstbegins.

The repeller grid next decreases in potential by AE and all switchestake the opposite state. The 5 cycle reference switch causes amplifier91 to store the ion current input signal it sampled before the switchingaction took place. The stored ion current signal is subtracted from thenew ion current signal by amplifier 97. This difference, AI, is

applied to switch amplifier 88, which is now Set, and its output is fedto the sample and hold amplifier 93. The output of amplifier 93 isapplied to the y axis of an x-y recorder. Each new difference is thenimmediately plotted in succession and held until the next cycle occurs.This process continues until the scan of all 100 steps of the electronenergy is completed. The x axis of the plotter obtains its signal fromdigital to analog converter 37. The resulting graph is AI vs. electronvoltage.

OPERATION The overall operation of the invention may be best understoodwith reference to FIGURE 1. The mass spectrometer is turned on by switchmeans not shown. With the spectrometer on, frequency divider 35 is fedthe operational frequency (10 kc.) and divides it into 5 cps. at itsoutput 110. However, the system does not start until the digital toanalog converter 37 is switched on (by switch 42, FIGURE 2). Whenconverter 37 is switched on, a staircase output is produced at itsoutput 113. This output 113 is fed to electron energy driver 51, controlgrid driver 59, and repeller grid driver 65; therefore keeping therelative voltage difference between the cathode and the grids constant,while raising the voltage level of the cathode. This means that, due tothe converter, the electron energy in the ionization chamber willincrease in proportion to the staircase wave from the output ofconverter 37. However, at the same time the repeller grid driver 65 hasa constant bias voltage switched in and out of its circuit at twice thefrequency of the staircase output. This causes the relative voltagebetween the cathode and the repeller grid to have a first and a secondvalue during each staircase level. This means that the electron energywill also have two values during each staircase level (E and E-AE; seeFIGURE 5C); where AE is a constant value.

Grid bias unit 63 is pulsed at 10 kc. to cause control grid driver 59 toalternate between driving the grid to a positive value set by thestaircase output and a negative value. When the control grid isnegative, no current fiows;

therefore, the electron energy supply to the ionization chamber is apulsed supply at 10 kc. The trap current regulator 57 senses the trapcurrent output of the spectrometer and provides a control output signal115 in order to regulate the trap current to a constant value.

The output of spectrometer 10 is a current output which represents theion current produced in the spectrometer. The spectrometer will havefour thousand pulse trains at its output at each staircase level. Twothousand pulse trains will be due to the ion current caused by electronenergy E and the other two thousand will be `due to electron energyE-AE, as explained above. The data processor 79 receives the two sets ofpulses, sums each of them and subtracts one sum from the other. Theoutput of the data processor, therefore, represents the ion currentdifference, AI, at each staircase voltage level. This is fed into the yaxis of x-y plotter 117. The x axis of the x-y plotter is fed thestaircase voltage out of converter 37; therefore giving a AI vs.electron energy plot. Other read-out devices could be used in place ofthe x-y plotter. For example, an analog to digital converter could beused so that the information would be in a form directly readable by acomputer.

The output of the spectrometer is also fed to an analog output unit 121.Unit 121 provides measurements of mass peaks representing either more orless than one ion per cycle. Its output is fed to an oscilloscope 122,.By the control of the units gating circuit, not shown, any of the masspeaks may be monitored by the oscilloscope. A detailed description ofanalog output unit 121 is not given as its details form no part of theinvention. The unit is the Analog Output System for the Bendix MassSpectrometer, Model 14 Series. Also, the total mass spectrum may beviewed on oscilloscope 124 which is connected directly to the output ofthe spectrometer.

A preferred embodiment of the invention has been chosen for purposes ofillustration and description. The preferred embodiment illustrated isnot intended to be exhaustive nor to limit the invention to the preciseform disclosed. It is chosen and described in order to best explain theprinciples of the invention and their application in practical use tothereby enable others skilled in the art to best utilize the inventionin various embodiments and modifications as are best adapted to theparticular use contemplated. It will be apparent to those skilled in theart that changes may be made in the form of the apparatus disclosedwithout departing from the spirit of the invention as set forth in thedisclosure, and that in some cases certain features of the invention maysometimes be used to advantage without a corresponding use of otherfeatures. It is, therefore, to be understood that within the scope ofthe appended claims the invention may be practiced otherwise than asspecifically described. Accordingly, it is desired that the scope of theinvention be limited only by the appended claims.

We claim:

1. A control system comprising first, second, and third voltage drivingmeans each having an output and at least one control input; each of saiddriving means having a bias voltage input; voltage stepping means havingan output connected to said control inputs so that each of said drivingmeans will have an output equal to the Sum of the voltage output of thevoltage stepping means and its bias voltage; an electron beam producingmeans having a cathode, first grid, second grid, and an anode; saidfirst driving means being connected to said cathode so as to supply it;said second and third driving means being connected to said first andsecond grids respectively so as to control the operation of the electronbeam producing means; the bias voltage of said second driving means ispulsed at a selected first frequency so as to cause a beam produced bysaid electron beam producing means to be pulsed at the same frequency; afrequency divider means connected to sense said first frequency and toproduce an output which is voltage having a Second frequency which is aselected division of said first frequency; and means connecting theoutput of said frequency divider means to a control input of saidvoltage stepping means whereby said stepping means has a staircasevoltage output which is stepped at a frequency proportional to saidsecond frequency.

2. A control system as set forth in claim 1, wherein said first grid isa control grid; and wherein said second grid is a repeller grid.

3. A control system as set forth in claim 2, wherein the output of saidfrequency divider means is connected to said third voltage driving meansso as to cause its bias voltage to be switched in and out of its circuitas said second frequency.

`4. A control system as set forth in claim 3, further comprising aregulator connected to the beam producing means so as to sense thecurrent output of the beam; said regulator having an output connected tosaid first voltage driving means so as to keep the beam currentconstant.

5. A control system as set forth in claim 4, wherein said beam producingmeans is set in an ionization chamber of a mass spectrometer; and saidbeam causes the ionization of a gas which is to be analyzed.

6. A control system as set forth in claim 5, wherein said massspectrometer has means therein whereby an output signal is producedwhich is representative of the number of ions produced by each pulse ofsaid beam and is at the same frequency as the pulsed beam; a dataprocessor; and said signal being applied to an input terminal of saiddata processor.

7. A control system as set forth in claim 6, wherein said data processorcomprises a first sample and hol circuit which has an input connected toan output of a first switching means by way of an inverter means; saidswitching means being switched at the frequency of said frequencydivider means by connections thereto; said switching means having aninput connected to said input terminal of the data processor; anamplifier having an input connected to the output of said sample andhold circuit, and an output connected to an output terminal of the dataprocessor; and connecting means connecting the input terminal of thedata processor to the input of said amplifier whereby its total inputwill be zero when the switching means is on and will be the differencebetween the signals present value and its last Value when the switchingmeans is off.

8. A control system as set forth in claim 7, wherein said data processorfurther comprises a second switching means and a second sample and holdcircuit; said second switching means being connected between the outputof said amplifier and an input of said second sample and hold circuit;said second switching means being switched at the frequency of thefrequency divider means and is phased with respect to the firstswitching means such that it is in an on condition when the firstswitching means is in an off condition; and an output of said secondsample and hold circuit being connected to said output terminal of thedata processor.

9. A control system as set forth in claim 8, further comprising an x-yplotter wherein its x input is connected to the output of said voltagestepping means, and its y input is connected to the output terminal ofsaid data processor.

10. In a data processor having an input terminal which receives a signalhaving a frequency which is greater than the frequency of its change ininformation; the improvement comprising an inverter means; a switchingmeans; a first sample and hold circuit which has an input connected toan output of said switching means by way of sald inverter means; saidswitching means being switched at the frequency of the change ofinformation and having an input connected to said input terminal; anamplifier havlng inputs connected to said sample and hold circuit, andan output connected to an output terminal of the data processor; andconnecting means connecting the input terminal of the data processor tothe inputs of said amplifier whereby its inputs will be equal andopposite when the switching means is on and whereby the amphfiers inputswill be the difference between the signals present information value andits last information value when the switching means is off.

11. data processor as set forth in claim 10, further comprising a secondswitching means and a second sample and hold circuit; said secondswitching means being connected between the output of said amplifier andan input of said second sample and hold circuit; said second switchingmeans being switched at the frequency of the change of information andbeing phased with respect to the first mentioned switching means suchthat it is in an on condition when the first mentioned switching means1s in an off condition; and an output of said second sample and holdcircuit being connected to sald output terminal of said data processor.

References Cited UNITED STATES PATENTS 2,810,075 10/1957 Hall et alZ50-41.9 3,012,139 12/1961 Hanson et al. Z50-41.9 3,154,747 10/l964Kendall Z50- 41.9 X 3,307,033 2./1967 Vestal 250-41.9

WILLIAM F. LINDQUIST, Primary Examiner U.S. Cl. X.R.

